An n.sup.+ -p-.pi.-p.sup.+ avalanche photodiode (APD) comprises a body of .pi.-type conductivity silicon (Si) having an n-type conductivity region extending a distance into the body from a portion of a first surface thereof with a p-type conductivity region extending a further distance into the body from the n-type region, and a p-n junction therebetween. A p.sup.+ -type conductivity region extends a distance into the body from a surface opposed to the first surface. Electrical contact is made to the n.sup.+ - and p.sup.+ -type regions.
In the operation of this APD, a reverse bias is applied to the electrical contacts producing an electric field within the APD whose profile depends upon the impurity concentration in the different regions and which forms a depletion region typically reaching through the .pi.-type region. Light incident on the surface containing the p.sup.+ -type region enters the photodiode and is absorbed primarily in the .pi.- or p-type regions, generating electron-hole pairs. The electrons are accelerated by the electric field until they attain sufficient energy for multiplication which typically occurs within one to three micrometers (.mu.m) of the p-n junction. Holes generated within the high field region are accelerated in the opposite direction and can also undergo multiplication where the electric field is sufficiently high.
One of the limitations of such an APD is that the multiplication process is noisy due to the width of the probability distribution of gains that a carrier can undergo. Webb et al, in RCA Review 35, 234 (1974) disclose that, to a good approximation, the excess noise factor F, defined as F=&lt;M.sup.2 &gt;/&lt;M&gt;.sup.2, can be expressed by EQU F=k.sub.eff &lt;M&gt;+(1-k.sub.eff)(2-1/&lt;M&gt;)
where k.sub.eff is a weighted average of the ratio of the hole ionization coefficient to the electron ionization coefficient, &lt;M&gt; is the average avalanche gain, and &lt;M2&gt; is the average value of the square of the gain. For a Si APD a typical value for k.sub.eff is between about 0.015 and about 0.1 and depends strongly upon both the electric field and its profile. To minimize the excess noise, k.sub.eff must be as low as possible; i.e., the hole multiplication must be minimized and the electron multiplication must be maximized. Because the ratio of the ionization coefficients decreases with decreasing electric field, the electric field should be large where the electron current is highest and hole current is lowest while the field should be small where the hole current is highest.
U.S. Pat. No. 4,463,368 issued to McIntyre et al on July 31, 1984 discloses an APD structure having a low value of k.sub.eff in the order of 0.006 to 0.008. This patent discloses an n.sup.+ -p-.pi.-hu +APD having an n.sup.+ -type region extending into the .pi.-type Si body a distance less than about 10 .mu.m and a p-type region containing acceptors in an uncompensated excess concentration corresponding to a dose of between about 1.times.10.sup.12 and about 3.times.10.sup.12 acceptors/cm.sup.2 of the surface area of the body into which the acceptors are introduced and extending into the body a distance in an APD where the multiplication occurs primarily at one end of a fully-depleted structure leaving a drift region at the other end in which the electric field is adequate only for rapid charge collection and does not contribute to the multiplication process. This APD is typically in the order of 100 .mu.m in thickness and the p-type impurity concentration region is diffused into the APD a distance of about 30 to 40% of the total thickness of the APD.